In the medical field, the use is known of special devices to detect the electrical activity behind heart function, both for diagnostic purposes and for simple purposes of heart activity monitoring.
Generally, such devices are provided with at least three electrodes to be positioned on the patient's body in order to detect the differences in potential which are created during cardiac activity.
The electrodes are positioned to form a triangle (Einthoven triangle) and are operatively linked to processing means adapted to graphically plot the pattern of the signal relating to the electrical activity of the heart over time.
The resulting pattern is known as electrocardiogram (ECG) and provides useful information on the patient's health condition, particularly related to health conditions tied to heart activity.
For each detection operation, three different ECG strips are obtained, these being the projection of the resulting cardiac activity on the three directions identified by the sides of the Einthoven triangle.
In particular, reference is made to the first derivation for the ECG strip relating to the right shoulder-left shoulder direction; to the second derivation for the ECG strip relating to the left inguinal-right shoulder direction; to the third derivation for the ECG strip relating to the left inguinal-left shoulder direction. By way of example, an ECG strip, referring to a cardiac cycle of a patient not subject to fibrillation, in first and second derivation, has the following major points:
wave P: this is the first section of the ECG which has a relative maximum point, conventionally positive with respect to the line, unlike zero potential difference defined as “isoelectric”, and refers to the wave and originates in the sino-atrial node and points to the depolarization of the atria;
group QRS: this is a complex of three successive waves reflecting the progressive depolarization of the ventricles, with the depolarization wave passing from the atrium-ventricular node to the surface of the ventricles.
The Q wave is negative with respect to the isoelectric and corresponds to the depolarization of the interventricular septum; the wave R is positive and has the maximum adjustable peak and corresponds to the depolarization of the left ventricle apex; the wave S is negative and corresponds to the depolarization of the part of the ventricles in contact with the atria;
wave T: this has a positive relative maximum and corresponds to the phase of re-polarization of the ventricle cells.
In third derivation, the ECG strip shows differences in representation due to the variability of the vector representing electrical activity, with the wave T, for example, which is negative.
In any case, a distinction can be made between the same major points.
From an examination of the ECG strips, the medical staff is able to obtain information on the patient's health, or to obtain useful information of various types.
The use is in fact known of the devices described above, for the sake of simplicity called ECG devices, as support instruments in the implanting of central venous catheters (CVC), of totally implantable systems and of catheters for hemodialysis.
The CVC implant in fact envisages the insertion of the catheter into a blood vessel making it run along this until the ending part of the catheter itself is positioned at the cavoatrial junction.
The most commonly used technique, to date, envisages the use of an ECG device connected to the ending part of the catheter, the latter used as an electrode to detect the differences in potential, so as to “guide” the doctor in the positioning of the ending part itself.
As the ending part of the catheter is pushed towards the cavoatrial junction, the ECG strip undergoes changes due to the change in position of the electrode with respect to the catheter.
According to established practice, doctors make reference to changes in the height of the wave P, considering as point of arrival the positioning whereby the ECG strip shows the maximum value of such height.
In detail, the doctor inserts the catheter and pushes it with gradual and predefined forward movements towards the cavoatrial junction, observing the ECG strip for each forward movement.
On the basis of his/her experience, the doctor checks the pattern of the wave P and establishes the final position of the catheter.
Usually, when the wave P starts to decrease, or even shows an initial negative section, the doctor returns to the immediately preceding position, considering him/herself to be close to the cavoatrial junction.
This known technique has a number of drawbacks tied to the fact that most of the work is entrusted to the doctor's experience.
The chosen final position may not be the ideal one, inasmuch as many factors accidentally influence the correct detection of the electrical activity of the heart. In order to address this type of drawback, proceeding is known with the aid of a radiographic control adapted to detect the position of the ending part of the catheter by X rays.
This solution too has a number of drawbacks tied to exposure to the X rays, known to be harmful for human health, and to the difficulty in controlling the position when the ending part of the catheter is covered by bone parts.